Sewing machines for attaching welts to pockets generally include a pair of sewing needles and a plurality of cutting edges for forming a pocket opening with V-shaped corner incisions as embodied by Durkopp machines 238N-313105, 238N-513105, 746-7/E13 and 746-7E40 shown in sections 31-34 of the 1979 Durkopp catalogue. A welt is fed to a sewing station at the machine by a device for edge-parallel folding of a fabric strip. Usually, the beginning and the end of a welt seam is marked on the material to be sewn.
The main disadvantage of this technique is due to the necessity for decreasing the stitching speed to enable the sewing-machine operator to stop the machine at the marked end location.
There are devices, as indicated in the above-mentioned Durkopp catalogue sections, for at least semi-automatically controlling a final stitching to a pocket of a welt having an attached pocket flap, such devices including a reflected-light detector for monitoring the passage of a back edge of the welt flap during feeding of the welt to the sewing station. Upon detecting a flap end, the detector activates a stitch counter which arrests the sewing-needle drive upon reaching a predetermined stitch count. Such a count is generally small, which condition facilitates the rapid sewing of a welt seam to a terminal stitch at a precise location relative to the edges of the welt. A disadvantage of this technique is that it may be not applied to flapless welts without incurring significant deviations in the ending points of the welt seams. Because a flapless welt does not lend itself to accurate monitoring by a reflected-light detector, the counter must number the stitches for the entire length of a seam, whereby errors arise from such sources as varying feed rates.